Display Technologies for Expanding the Pharmaceutical Applications of Cyclotides

Cyclotides are ultra‐stable peptides originally discovered in plants based on their medicinal applications. Their natural function is as host defence agents. They are amenable to chemical synthesis for use as scaffolds for drug design applications. Cyclotides comprise ~30 amino acids and in addition to having a head‐to‐tail cyclic backbone, incorporate six conserved cystine residues connected in a cystine knot motif. The cyclic backbone and cystine knot contribute to their exceptional resistance to proteases or thermal denaturation, making them useful scaffolds for drug design applications. The backbone segments, or loops, between the conserved cysteine residues are amenable to combinatorial variation in native cyclotides and have also been used to incorporate selected bioactive peptide epitopes into a range of synthetic cyclotides and cyclotide‐like scaffolds. In the past this was largely done via low throughput structure‐based design approaches, but the discovery of novel cyclotide binders has been greatly enhanced by the use of combinatorial display approaches on cyclotide scaffolds using phage, bacterial, yeast and mRNA technologies, as reviewed herein.